Atomic absorption spectrophotometry (AAS) is a well-known technique for quantitative determination of an analyte element in a liquid sample. AAS makes use of the fact that atoms of an element absorb radiation in spectral lines of the same frequency as emitted by the element when subjected to appropriate stimulus. Accordingly, a beam of radiation containing the frequency absorbed by the element sought to be determined is passed through a sample and the degree of attenuation of the beam measured by a suitable detector which generates an electrical signal representative of absorption of the beam by the sample which is, at least theoretically, a function of the concentration of the analyte.
The application of this phenomenon of course requires that the sample be in atomic form. While atomization can be accomplished by directing a nebulized quantity of the liquid sample into the flame of a specially designed burner, the present invention is concerned with electrothermal or "flameless" atomizing methods and apparatus. Conventionally, electrothermal atomization employs an electric furnace consisting primarily of a graphite tube having a sample port in its side wall at the midpoint of its length. The furnance tube is mounted between electrodes engaging its ends. The electrodes are annular in form in order to accommodate passage of the spectral radiation beam emitted from a suitable source such as a hollow cathode lamp (HLC) or electrodeless discharge lamp (EDL).
The furnace tube is heated by passing an electric current longitudinally therethrough between the annular electrodes. The customary analytical procedure consists of introducing a small quantity of the liquid sample into the tube by way of the sample port and applying a relatively low current sufficient to heat the tube to a temperature sufficient to volatilize the solvent components, i.e., the drying temperature. The vaporized substances are carried off by means of a flow of inert gas through the tube. The electric current is then increased to the ashing stage, producing a temperature to cause chemical decomposition of the sample. Finally, the heating current is raised to an intensity such as to achieve a temperature effective to atomize the sample, producing a "cloud of atoms" in the furnace tube.
It is desirable to delay the atomization of the sample until the entire inner wall surface of the furnace tube attains atomization temperature. To this end, a small, essentially planar graphite member, known as a "sample platform", is placed in the tube at the sample introduction site. The platform is constructed and arranged so that conduction heating is minimized and the platform is heated substantially entirely by radiation from the walls of the tube.
The sample is introduced through the sample port and deposited on the platform. This arrangement causes a time lag in the heating of the platform delaying the volatilization and vaporization of the sample until the tube and the gasses have reached temperature equilibrium. A reduction in interferences is achieved by means of the platform furnace; however, the quantity of sample which a platform small enough to yield the benefits sought is quite limited. Larger samples are advantageous as the result in higher sensitivity.
For additional details regarding the platform furnace reference may be had to Spectrochimica Acta, Vol. 33B, 1978, pp. 153-159; DE-C2-29 24 123.
A recent development in the field of the invention is a method and apparatus for atomizing a sample wherein a "thermospray" vaporizing device is used. A carrier liquid such as de-ionized water is pumped through a heated capillary tube made of fused silica. The capillary tube is encased in a stainless steel tube and is axially displaceable. The stainless steel tube is heated by passing through it a high intensity electric current. Concomitantly the stainless steel tube heats the silica capillary. In its passage through the tube, the carrier liquid is at least partially vaporized and emerges from the outlet end of the tube in the form of a vapor spray.
The flow path of the liquid carrier includes, upstream of the capillary, a loop containing the sample and an injection valve operative selectively to bypass or include the loop in the flow path. When the loop is coupled in the flow path, the sample is entrained by the carrier liquid and passed through the capillary tube.
The axially displaceable tube has a first, retracted limit position and a second advanced limit position. In the advanced position the outlet end of the capillary tube extends into a graphite furnace tube of the type described above through the lateral sample introduction port in its side wall. The jet emerging from the capillary tube is directed to impinge on the inner wall of the furnace tube opposite the sample port.
In the retracted position, the outlet end of the capillary tube is disposed within a vacuum exhaust chamber, from which the spray emerging from the capillary tube is exhausted. A timer synchronizes the coupling of the sampling loop into the carrier liquid flow path and the movement of the outlet end of the capillary tube into the furnace. Thus, the capillary tube is initially in its retracted, position and water spray, issuing from the outlet end of the heated capillary tube is exhausted through the vacuum chamber. The loop is then coupled into the carrier liquid flow path by operation of the sample injection valve; the sample is entrained by the carrier liquid, carried through the capillary, and vaporized. Contemporaneously the outlet end is moved into the furnace, which is maintained at the relatively low drying temperature. The sample components of interest are deposited on the inner wall of the furnace tube while the vehicle and solvent vapor is removed by flow of inert gas through the furnace.
The furnace is then heated to atomizing temperature and the absorption of the beam of radiation by the cloud of atoms is measured as previously explained. As there is no need to accommodate all of the solvent within the furnace, much larger sample quantities can be used so that the sensitivity of the measurement is enhanced. Moreover, inasmuch as the furnace need not be cooled down to ambient temperature after each analysis but remains at the elevated level of the drying temperature, the analytical cycle time is reduced considerably. During the heating of the furnace to atomizing temperature, a shield is inserted between the vacuum exhaust chamber and the furnace in order to protect the hot furnace from the spray emerging from the capillary tube.